Computers and related electronic equipment typically have a number of different types of data storage or memory devices. For example, a read only memory (ROM) is a semiconductor memory device in which data is permanently stored. The data cannot be overwritten or otherwise altered by the user. A ROM is also non-volatile which means that the data is not destroyed when power is lost. A ROM is "programmed" during its manufacture by making permanent electrical connections in selected memory cells. ROMs are useful wherever unalterable data or instructions are required. A disadvantage of ROMs is that their programming is determined during the design stage and can only be varied by redesigning the chip.
An alternative to a ROM is a programmable read only memory (PROM) which is programmable once after its manufacture. In one type of PROM, each memory cell is provided with an electrical connection in the form of a fusible link. The PROM is "programmed" by blowing the fusible link in selected cells. A PROM is frequently programmed at the factory; however, it is not uncommon for it to be programmed by the purchaser. A clear advantage of a PROM is that a single semiconductor design can have many different applications.
There are a considerable number of known designs for the fusible links used in PROMs. Perhaps the most common is a metal link, such as tungsten, which is narrowed or necked down in one region. To blow the fuse, current is driven through the link. The current heats the link to its melting point and the link is broken. Usually the link breaks in the necked down region because that is where the current density is highest.
One problem with the design of existing fusible links is that they usually call for a relatively high programming current. The requirement for high current can affect the basic manufacturing process for the PROM. For example, a popular process is one which produces MOS transistors in a CMOS configuration. Another process is one which produces bipolar transistors. It is costly to integrate CMOS and bipolar designs into a single process so it is generally preferred to use either a CMOS or bipolar process, but not both. To obtain the high current required to blow a fuse, generally bipolar technology is required. This means that a CMOS PROM design will require bipolar transistors to drive the programming current.
Another problem with existing fuse designs is that certain fuse metals, such as tungsten, require a specialized process to properly deposit them on a chip. As with bipolar transistors, different or specialized processes increase the cost of chip manufacture.
A further problem with existing fuse designs is referred to as "grow back" or "metal migration". Grow back is a phenomenon in which a fuse metal which has been blown grows back together to reconnect the fusible link. Although not completely understood, grow back alters the data stored in the PROM--clearly an undesirable result.